Project Summary Animals must cope with the pervasive force of gravity as they navigate the environment. To sense and respond to this force, vertebrates rely on signals from the inner ear, where gravito-inertial sensors called otoliths drive activity in peripheral vestibular circuits. This information is then processed by central vestibular neurons in the brainstem and transformed into postural outputs via projections to the spinal cord. Because this vestibulospinal circuit is formed early in life, it has been technically challenging to examine how it develops. The objective of this proposal is to determine how sensory computations arise in vestibulospinal neurons, and whether normal sensory activity is required for this development. To surmount the technical challenges of examining these circuits at early stages, I propose to use the larval zebrafish. Zebrafish are an excellent system for this line of research because of their accessibility, transparency, and homology to other vertebrates. Furthermore, we can carry out many experiments that are not feasible in mammalian models, including in vivo whole cell patch-clamp analysis of synaptic responses to sensory stimuli. This technical advance permits us to record sensory-evoked activity in the intact brain, over the time period when postural behaviors develop. In addition, we can exploit a mutant fish line in which otolith development is delayed by two weeks, providing in effect a reversible sensory deprivation to vestibular circuits. The proposed experiments will therefore reveal how sensory information is encoded during development, both under normal conditions and those of sensory delay. In Aim 1, we will use a combination of behavior, imaging, and physiology to define the anatomy, sensory responses, and functional role of vestibulospinal neurons in vivo. These experiments will identify anatomically distinctive vestibulospinal neurons for characterization across animals. In Aim 2, we will examine the development of vestibular encoding during the time period in which animals begin to self-right with respect to gravity. Here we will use both anatomical imaging of vestibular afferents to the central vestibulospinal neurons as well as physiological analyses of the development of sensory encoding. Finally, in Aim 3 we will use an animal model of sensory deprivation to ask whether the first two weeks represent a critical period in vestibular development: does the vestibular circuit develop normally in the absence of normally patterned sensory input? Because this model develops an otolith on a grossly delayed time scale, it serves as a model of reversible sensory deprivation so that we can also examine whether and how the circuit recovers. Together, the proposed experiments will provide a view of vestibular development under both normal and abnormal sensory conditions.